The present invention relates generally to the field of storage and retrieval of data within magnetic media. In particular, the present invention relates to high magnetic anisotropy in hard magnetic bias elements of magnetoresistive sensing elements.
A transducing head of a magnetic data storage and retrieval system typically includes a magnetoresistive (MR) reader portion for retrieving magnetic data stored on a magnetic media. The reader is formed of multiple layers which include an MR sensor generally positioned between two insulating layers, which are in turn positioned between two shield layers. The MR sensor may be any one of a plurality of MR-type sensors, including, but not limited to, anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), tunneling giant magnetoresistive (TMR), spin-valve, and spin-tunnel sensors.
When the transducing head is placed near a magnetic medium, resistance of the MR sensor changes in response to a magnetic field emanating from written transitions in the magnetic medium. By providing a sense current through the MR sensor, resistance of the sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium.
To operate the MR sensor properly, the sensor must be stabilized against the formation of edge domains because domain wall motion results in electrical noise that makes data recovery difficult. A common way to stabilize the sensor element as a single domain is by applying a magnetic bias in a desired direction. Thin films of hard magnetic materials have been used in various types of sensing elements including AMR, GMR, TMR, etc., to bias the magnetic field sensing layer. One common design is a “permanent magnet abutted junction” where permanent magnet (PM) bias elements directly abut opposite sides of the sensor element. The permanent magnets have a high coercive field and remanent magnetization oriented in a desired direction, for example parallel to the air bearing surface (ABS) of the sensor element.
During fabrication of the sensor element, the permanent bias elements are formed by depositing hard magnetic materials in a thin film formed of multiple crystalline grains. The magnetic properties of the hard magnetic thin film are influenced by the formation of magnetic domains, which may be referred to as magnetic grains. Each magnetic domain includes one or more crystalline grains having a magnetic moment. The magnetic moments lie generally in the plane of the film. However, within the film plane the magnetic moments are randomly distributed. The result is a hard magnetic thin film with no net in-plane anisotropy.
To properly function as a biasing element, the hard magnetic material must have its remanent magnetization orientated. The magnet is “set” by orienting the magnetic moment of each magnetic domain by applying a large magnetic field in the desired direction, for example parallel to the air bearing surface (ABS). Ideally the application of this field leads to an alignment of the magnetic moment of each individual grain such that all the moments are oriented parallel. Due to the random orientation of each grain's magnetic moment in the unset film, some grains resist the setting field more than others and can produce less than perfect alignment of the magnetic moments within the hard magnetic thin film.
The size of sensor elements in transducing heads is shrinking in response to the increasing areal density of magnetic media. The result of decreasing sensor element size is fewer “magnetic grains per sensor” in the hard bias elements. The sensor element dimension perpendicular to the ABS, commonly known as the stripe height, is currently approaching 100 nanometers or less and is likely to continue to diminish. Dimensions of that magnitude can lead to a few as 1 to 3 magnetic grains of hard magnetic material per junction in the stripe height direction. Thus when a small sensor has even one misoriented magnetic grain, a larger number of magnetic grains per sensor are oriented in an errant direction.
Imperfect alignment within the hard magnetic thin film of the biasing elements can lead to a degradation of sensor properties, including head-to-head comparisons of amplitude, noise and glitching. Because recording heads are fabricated en masse on a single wafer it is desired that any variation in operation between heads is minimized or ideally is zero. However, distortion caused by misalignment of single grains within the hard magnetic bias elements increases head-to-head variation. The result is reduced manufacturing yield, increased production cost and potentially increased field failure rates. Consequently, there remains a need in the art for improved magnetic anisotropy in permanent magnet bias elements thereby reducing distortion in sensor properties.